The present invention relates to electronic devices, and, more particularly, to power supply and control as may used in portable computer systems.
Laptop and Smaller Computers
Portable personal computers were introduced in the early 1980s and proved to be very useful and popular. As this market has developed, it has become increasingly clear that users strongly desire systems to have small volume, small weight, physical durability, and long battery-powered lifetime. Thus, small portable computers ("laptop" computers) proved to be extremely popular during the late 1980s. Users continue to demand more features, longer time between battery recharges, and lower weight and volume. This combination of demands is difficult to meet. Moreover, as of 1990, another smaller generation of portable computers has begun to appear, referred to as "notebook" computers. This smaller form factor will only exacerbate the difficulty of the above tradeoffs.
Approaches to Power Conservation
There are basic approaches to extending the operating lifetime of a portable computer. The simplest way is to specify components at the lowest economical power consumption. Thus, for instance, CMOS integrated circuits and liquid crystal displays (LCDs) will normally be used.
An equally simple way is to increase battery capacity. However, both of these routes rapidly encounter limits, which are set simply by the tradeoff of the cost of lower-power components, or of the elimination of functionality, with user expectations.
Another approach focusses on the conversion efficiency of the battery power to the regulated AC and DC power actually consumed by the computer components.
And a fourth way invokes power-management algorithms so that, at almost every instant, all components are being operated in the lowest-power mode for their current demands. Thus, for example, a processor which is not currently executing a program may be placed into "sleep" mode, to reduce its overall power consumption. For another example, substantial power savings can be achieved simply by stopping the system clock. For another example, it is common practice, in portable computers with an LCD display, to provide backlighting for use of the display under low-light conditions. Because this backlighting consumes relatively large amounts of power, it will normally be turned off after a short period of inactivity (or even, alternatively, after a short duration regardless of activity), until the user again demands backlighting.
All of these lines of approach have some inherent limits. For example, it is hard to foresee any integrated circuit technology which would be economical in the 1990s and more power-efficient than low-power low-voltage CMOS. Some further improvement in this area is foreseeable, but no revolutionary improvements appear likely. Moreover, in practice, such improvements are largely outside the control of system designers: when lower-power chips are sampled, system design houses will buy them; but system design houses cannot greatly accelerate the pace of introduction of such chips.
It is also true that the smartest power-management programs cannot reduce the time fraction during which the user wishes to look at the display, or enter data through the keyboard. However, in this area there does appear to be room for improvement, and system design improvements can help achieve power efficiency.
Many power management schemes have been proposed, where parts of the system are shut down during periods of inactivity. These approaches tend to extend the usable working time between recharges.
In addition, it has been recognized that management of the charging and discharging cycles of Ni-Cd or NiMH.sub.4 batteries can help to extend their life.
Either of these power-management functions requires some intelligent control. The conventional way to implement this has been using the main microprocessor (CPU). To accomplish this, the necessary program steps are inserted into the BIOS software (basic input/output system software), which is stored in ROM.
Display Power Consumption
FIG. 1 illustrates in perspective view a notebook type portable computer system labelled with reference numeral 100. System 100 typically has a seven volt main battery made of six 1.2 volt rechargeable nickel-cadmium cells in series and a battery power manager for supplying the various power needs, such as 5 volts regulated DC, 12 volts regulated DC, and high voltage AC to power the backlighting for the display. The display typically utilizes reflective transmissive or transflective technology to achieve brightness and contrast. Both transmissive and transflective displays employ some form of backlighting. Cold cathode fluorescent lamps (CCFL) provide the highest available efficiency for illumination of the display. The backlighting of the display typically consumes about half of the battery life for a notebook type portable computer and thus the efficiency of the backlighting greatly impacts battery life.
A CCFL requires high voltage AC to operate: about 1500 volts to strike the arc and about 300 volts to sustain it. Thus the power manager typically includes a DC to AC inverter which may be of the type shown schematically in FIG. 2. In addition to high efficiency (for long battery life), the inverter should deliver a sinusoidal lamp drive current to minimize RF emissions. However, known low voltage inverters only achieve efficiencies of 80% or less and also need component changes to accommodate variant CCFL configurations such as additional parallel or series lamps. Inverters based on silicon controlled rectifiers (SCRs) provide higher efficiencies but other problems. Generally, see Pressman, Switching Power Supply Design (McGraw-Hill 1991).
Inverters
The prior art inverter of FIG. 2 operates as follows. Vin connects to a low voltage DC source, such as a battery at 7 volts, and pass FET Q1 together with free-wheeling Schottky diode D1, inductor L1, plus capacitor C1 form a buck regulator to step Vin down to Vreg=Vin.times.the duty cycle of pass FET Q1. Pulse width modulator PWM samples the current through cold cathode fluorescent lamp CCFL-1 by tapping resistor R3 and uses this feedback to drive the gate of pass FET Q1 to control its duty cycle and thus the voltage delivered; the pulse frequency of PWM is selected high enough to permit L1-C1 to lowpass filter the output. The base-tied npn transistors Q2 and Q3 alternatively conduct current due to the positive feedback between increasing collector current and increasing base current by the inductive couplings of the three primary windings T1-P-1, T1-P-2, and T1-P-3 of transformer T1; this is essentially a Royer oscillator. In particular, if Q2 turns on with a small base current limited by resistor R1, then the collector current of Q2 will increase and the positive feedback from winding T1-P-1 to winding T1-P-3 will drive the base more positive (and the base of Q3 negative to keep Q3 turned off) and Q2 quickly turns fully on. Then Vreg appears mostly across winding T1-P-1 as the collector current ramps up. The core of transformer T1 may saturate as the collector current continues to increase. Now the gain (.beta.) of Q2 is collector current dependent and decreases for large currents, so the rate of increase of the collector current peaks and the voltage drop across winding T1-P-1 falls with the collector voltage of Q2 increasing up to Vreg. The dropping of the rate of increase of the collector current implies a drop in the positive feedback to the base of Q2, and this feeds back on itself to quickly turn off Q2 and drop the collector current. Thus the flux in tramformer T1 collapses (with circulating current through capacitor C2), which induces a negative bias at the base of Q2 but a positive bias at the base of Q3 to turn on Q3. And the increasing collector current through winding T1-P-2 will provide positive feedback to winding T1-P-3 to fully turn on Q3 analogous to the positive feedback for Q2 previously described. Again, the fi fall off will decrease the rate of collector current increase and the consequent drop in positive feedback will turn off Q3. And the turn off of Q3 analogously turns on Q2.
The windings T1-P-1 and T1-P-2 in transformer T1 have opposite orientations, so the secondary current through T1-S changes direction when Q3 turns on and Q2 turns off. The ratio of the number of turns in one of the primary windings carrying a collector current to the number of turns in the second winding together with the magnitude of Vreg determines the magnitude of the induced secondary voltage which applied across cold cathode fluorescent lamp CCFL-1. Thus to generate 1500 volts (with zero current) in the secondary and with a Vreg of about 5 volts in the primary, the winding ratio should be about 300 to 1 for secondary to primary. Capacitor C3 is much larger than the capacitance of CCFL-1 when CCFL-1 is not conducting, so the 1500 volts appear essentially across CCFL-1. Once CCFL-1 sustains an arc and provides only a 200-300 volt drop, capacitor C3 (plus the secondary winding resistance) provides the impedance for the remaining 1200-1300 volt drop in the secondary circuit.
Resistors R5, R3, and R4, diodes D2 and D3, and capacitor C4 provide a sampling of the CCFL current for feedback to pulse width modulator PWM. Indeed, diode D2 charges up capacitor C4 to the peak positive voltage across resistor R5, and resistors R3 and R4 provide a small leakage current from capacitor C4 and also provide a tap to pulse width modulator PWM. If the voltage in the secondary circuit is too high, then pulse width modulator PWM will lower the duty cycle of pass FET Q 1 and thereby lower Vreg which implies smaller collector currents and thus smaller secondary voltage.
Inverters such as in FIG. 2 typically have optimized switching transistor drive over the Vin range of operation. A problem with this approach lies with the regenerative base drive scheme and the use of bipolar transistors. Bipolar transistors require base drive current to be consumed from the feedback winding causing power losses. While these losses can be minimized at one particular input voltage, they begin to increase as Vin deviates from the optimization level. As Vin increases so does the base current thereby causing additional power consumption. With a constant power load as with the typical CCFL, the collector current decreases as Vin increases, therefore less base current can support the collector current. As Vin decreases from the optimization level the collector current increases to maintain a constant power load, and the base drive decreases. This decrease in base drive causes the switching transistors to incur additional saturation losses and lower system efficiency.
Such a base drive scheme limits the dynamic range of the input voltage Vin, and the need to carefully tune the transformers to the load. This tuning requirement limits the inverter's ability to accommodate different CCFL lamp types and configurations.
Features
The present invention provides an inverter for CCFL driving which includes logical switching of field effect transistors, nonsaturating transformers, plus a resonant LC circuit to achieve 90% efficiencies and can accommodate CCFLs in single or dual configurations without a change in circuit components.